I own both. Quality-wise, I haven't seen a noticeable difference, though I've never tested it using charts or any of that.

Which one do I use more? The pancake, but only because of its size. It's quite a versatile lens to work with and the J1 + 10mm combo lends itself well to being brought everywhere.

The only thing I don't like about the pancake is its MFD, which is almost the same as the 10-30 or even the new 18.5 prime. I like using it for closeups and wish at times that I could get a little closer with it.